Drought-Proof Design

Desalination offers alternative water supply

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Perhaps no one captured the vision as well as John F. Kennedy.

“If we could ever competitively, at a cheap rate, get freshwater from saltwater,” he said in a 1961 press conference, “it would be in the long-range interests of humanity, which would really dwarf any other scientific accomplishments. I am hopeful that we will intensify our efforts in that area.”

Kennedy’s vision for reliable, economical water supplies was soon eclipsed by his other calls to action, most notably his Moon Speech the following autumn, but now, more than 50 years later, the world is entering a new age of water—one to be driven by desalination technology on account of great need.

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Though water is abundant on Earth, most of it is unsuitable for human use. Some 97% is saltwater in the oceans, and of the remaining freshwater, over two-thirds, is frozen in glaciers and ice caps. Groundwater and surface water—the primary sources for municipal and industrial supply—make up less than 1% of Earth’s water. The most accessible and economical freshwater sources have already been developed, especially in water-scarce areas. Climate change and growing demands necessitate novel solutions.

Enter desalination. Having advanced tremendously in recent years, desalination—the process of transforming saline water to clean, drinkable water by removing salts—offers new alternatives. It expands the set of potential sources to include seawater, brackish groundwater, and other saline or contaminated water. Rather than be limited to a sliver of Earth’s already-stressed freshwater, water suppliers can use desalination to tap larger and more reliable sources.

“Desalination is one of several tools communities can use in appropriate circumstances to gain greater water security,” says Felicia Marcus, California Water Board Chair, in a news release.

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West reverse osmosis permeate collection tubing
West reverse osmosis permeate collection tubing

To be sure, desalination is no panacea. In most cases, it is a desperate last option. The process is energy intensive, requires special materials to handle corrosive outcomes of water chemistry, and has certain environmental impacts that must be weighed carefully. Some are concerned that desalination only encourages growth and development when wise resource management is needed most. Still, many are including desalination as part of a strategic water portfolio—a diverse set of water assets that collectively lowers risk and increases reliability.

“Desalination is typically employed when there is water scarcity,” says Ronan McGovern, a researcher at MIT’s Center for Clean Water and Clean Energy. “In such scenarios, the [alternatives] include pumping water over long distances, recycling wastewater, and increasing water conservation. There is no obvious winner as all have benefits and drawbacks. Water conservation measures can be politically difficult. Wastewater recycling is cost effective, but in periods of drought there is less wastewater to recycle. Pumping over long distances may require pipes to be laid in environmentally sensitive locations.” These constraints may leave desalination the most viable option.

“The benefits are that it creates a new source of water that is robust to drought, may be more environmentally acceptable than pumping water from afar, and may be more politically acceptable than water conservation,” says McGovern.

Almost all water has some salinity. Freshwater is defined as having a salt content, measured as total dissolved solids (TDS), of less than 0.05% (500 mg per L) by mass. Brackish water is saltier than freshwater but less salty than seawater, ranging from 0.05–3% (500–30,000 mg per L) TDS. Brackish water is found at freshwater–seawater interfaces such as estuaries and in many aquifers in the southwest United States, especially in Texas and New Mexico. Saline water ranges from 3–5% (30,000–50,000 mg per L) TDS and includes seawater with a typical salinity of 3.5% (35,000 mg per L). Water with salt concentrations over 5% (50,000 mg per L) TDS is called brine. Salinity in Utah’s Great Salt Lake, for example, ranges from 5–27% (50,000–270,000 mg per L) depending on the lake’s level. The higher the salinity, the more effort is required to desalinate.

About 16,000 desalination plants of various sizes operate around the world. Output has tripled since 2000, and construction is accelerating while technology improves. Global capacity in 2013 was estimated at 21.1 billion gallons per day, and 300 million people in 150 countries rely on desalinated water for some or all their daily needs, according to the International Desalination Association. Saudi Arabia is the largest desalinator, accounting for 17% of global desalination output, followed by the United Arab Emirates and the United States, which each make up 13%. Most desalination in the United States is for inland brackish groundwater, though seawater desalination will be the main source of desalination growth.

At least a dozen different desalination technologies exist. Distillation was the first—used thousands of years ago, and the most common through the 1990s. The process involves evaporating water by thermal or vacuum methods, thereby leaving solids behind, and condensing the desalted vapor into liquid form. Multi-stage flash, multiple-effect distillation, vapor distillation, and vapor compression are examples of such processes. Thermal distillation produces about half of all desalinated water in the world, but is being outpaced by more efficient methods.

Electrodialysis reversal (EDR) arose in the 1960s. During EDR, feed water passes through an electric field. Dissolved positive and negative ions (e.g., sodium and chloride) migrate through a stack of alternating ion-exchange membranes until completely separated from the water. Periodically, the direction of ion flow is reversed by switching the polarity of the applied electric current. While water does not pass through the membranes, EDR effectively removes a variety of constituents. Not all feed water can be converted to permeate, or water cleaned of most dissolved solids; some water is needed to dilute and flush the brine before discharge. Recovery from EDR can exceed 90% depending on the water chemistry.

An EDR facility in Magna, UT, treats slightly saline groundwater that is also contaminated with arsenic and perchlorates. The system, engineered by GE, produces 6 million gallons per day with an 85% recovery rate and a salinity reduction of 70%.

Reverse osmosis (RO) is the most common desalination technique today. Though not new—it was developed in the 1960s—the technology has improved in recent years to the point of being competitive with other options.

In RO, feed water is driven under high pressure through a semi-permeable membrane that separates salt and water. The membrane pores allow only water molecules to pass through while holding back larger salts, minerals, and other solids. The permeate is collected; the rejected brine, or concentrate, is routed through the next membrane unit to produce more permeate or is collected and discharged.

RO membranes are manufactured in sheets, which are wrapped around a collector tube to produce a cylindrical unit a few feet long. Feed water travels between the membrane layers along the cylinder, and the permeate migrates in a spiral direction to the core collector tube. Membranes are loaded into pressure vessels and arranged in racks with connecting tubing. The type, size, and number of RO units depend on the flow rate and water quality. Typical water recovery is about 80% in RO systems, though brine discharge may require dilution and therefore lower water recovery.

A $1 billion project in Carlsbad, CA, the flagship of modern desalination technologies and the largest desalination plant in the western hemisphere, was set to open in fall 2015. The Carlsbad desalination project will produce and deliver 50 million gallons per day of high-quality water to San Diego County. At least 15 other desalination projects are proposed along the California coast.

Where 80% of the county’s water is imported, the project aims to diversify its water sources, a long-term need underscored by four consecutive years of severe drought throughout the state. Some find it ironic that a state with more than 3,400 miles of coastline could be so dry.

Crews install piping around high-pressure pumps at the Carlsbad desalination project.
Crews install piping around high-pressure pumps at the Carlsbad desalination project.

“California is in a serious drought right now, and any new water supplies are important to the region,” says Bob Yamada, water resources manager for San Diego County Water Authority. “Unlike water that comes from rainfall or water that comes from snowpack, we’re utilizing what essentially is the world’s largest reservoir—the Pacific Ocean.”

The public–private partnership between the San Diego County Water Authority and development specialist firm Poseidon Water, employing IDE Technologies as a joint venture with Kiewit-Shea Desalination, will provide 7% of

the county’s water by 2020, or enough for 300,000 people. The desalinated water will be mixed with other sources and become part of the overall supply. “The Carlsbad project revolutionizes desalination in the United States,” says an IDE spokesperson.

More than 20 years in the making, the project includes an intake pump station and pipeline, desalination plant, product water storage, brine discharge pipeline, waste stream treatment facilities, product water pump station, and 10-mile delivery pipeline. Some of theinfrastructure overlaps with the adjacent Encina Power Station, which helps save costs.

Up to 100 million gallons per day of seawater used for once-through, non-contact cooling at the power station will be routed to the new desalination plant. The source water will be filtered to remove large particles and bacteria.

After pre-treatment, chemicals will be added and the feed water will undergo reverse osmosis through thousands of tubular membranes. Half of the water will become drinkable, and the remaining brine will be flushed out. The entire treatment process will take about 20 minutes. After chlorination and conditioning for taste, product water will be stored and delivered to a regional distribution system via a 10-mile pipeline.

An RO pilot plant has been onsite to gauge public acceptance, and over 99% of taste testers rated the quality as “good” to “excellent.”

The rejected brine must be handled carefully. Its discharge has required a 63-inch-diameter, specially designed high-density polyethylene (HDPE) pipeline, provided by ISCO Industries, to accommodate high velocities, steep slopes, and corrosive chemistry. Since the brine will have twice the salt content of the original seawater, it will be diluted with the power station’s cooling water return flow before being discharged to the ocean.

The project will use 40% less energy than conventional desalination processes, thanks to efficient pumps, advanced membranes, and energy recovery systems. “The Carlsbad plant will be the most energy-efficient and technologically advanced plant in the Western hemisphere when online this fall,” says Jessica H. Jones, community outreach manager at Poseidon Water.

While the product is more expensive—the desalinated water will cost $1,900–$2,200 per acre-foot depending on how much is purchased—it is much more dependable than current supplies. “The Carlsbad plant is the only local, reliable water supply in San Diego that is not dependent on rainfall or the snowpack,” says Jones. “It is drought proof.”

Far from any ocean, San Antonio, TX, is the site of a major inland desalination project.

In 2012, the San Antonio Water System (SAWS) launched a program to develop and desalinate brackish groundwater in southern Bexar County. The program will help meet water needs over the next 50 years as the city weans itself from heavy reliance on the Edwards aquifer, a prolific but sensitive groundwater system in which demand has exceeded capacity in recent decades. The Wilcox aquifer, by contrast, is largely untapped, and the Texas Water Development Board confirmed that a sufficient supply of brackish groundwater exists there.

“Utilizing brackish water allows our community to make the most of a previously untapped resource. San Antonio continues to invest in new sources,” says SAWS President and CEO Robert R. Puente in a news release. “Brackish groundwater is plentiful and unused in our region, and available for centuries. The State of Texas views desalination as a solution to meet future water demands, and so do we.”

Inland desalination presents some unique challenges. Unlike coastal plants, they may not have a consistent, ample water supply, nor can they discharge brine to the ocean.

The $400 million project consists of a well field, reverse-osmosis treatment, and concentrate disposal via underground injection. When complete in 2016, the project’s first phase will produce 10–12 million gallons per day. Black & Veatch is designing the facilities, and joint venture partner Zachry-Parsons is managing construction. Future phases through 2026 will increase capacity to 30 million gallons per day.

A similar project in El Paso, the Kay Bailey Hutchison Desalination Plant, has been operating since 2007. Where average precipitation is less than nine inches per year, the plant can produce 27.5 million gallons per day of potable water from otherwise unusable brackish groundwater in the Heuco Bolson aquifer. At the time, it was the world’s largest inland desalination plant. A public–public partnership of El Paso Water Utilities and the US Army’s Fort Bliss, the project augments water supplies in a region continually challenged by limitedfreshwater resources.

The facility withdraws water from 16 production wells and pre-treats it with sand filters, cartridge filters, and anti-scalant. Five two-stage RO trains with membranes by Hydranautics remove salts and produce 15 million gallons per day of permeate with an 83% recovery rate. The permeate is blended with 12.5 million gallons per day from blend wells before delivery. The brine is discharged via deep well injection 22 miles away. The project cost $90 million, with brine disposal making up $19 million. The facilities ensure El Paso a sufficient water supply for the next 50 years and beyond.

A rack of reverse osmosis vessels at the Carlsbad desalination project
A rack of reverse osmosis vessels at the Carlsbad desalination project

Like all water alternatives, desalination has its challenges. The major drawback of desalination is its immense energy consumption. Energy is the largest variable cost for a desalination plant, varying from one-third to more than one-half the cost of produced water. Desalinating seawater can consume 5–20 times as much energy as other water sources.

Technologies are improving, however, and are becoming more energy efficient. “Energy and costs have already fallen by more than 50% over the last two decades,” says McGovern. “The rate of improvement in the next decade will be slower, but perhaps we will see further improvements of about 10% in equipment and energy—driven by improved membrane chemistries and pump efficiencies.”

Among promising technologies are graphene membranes one atom thick that minimize resistance, and nanoengineered pores that improve flow. Though such wonders are many years away from commercial availability, research suggests energy reductions of 15–45% and entire plants at half the usual size. If scalable, these and other novel methods, such as electrochemically mediated desalination, could bring a long-needed breakthrough.

Even without one, though, there are already other ways to reduce the energy impacts. Efficient equipment is one starting point. Soft-start motors and variable-frequency drives can help regulate torque for varying flow rates and thereby avoid energy peaking or over-pressurization that may otherwise occur.

Beyond equipment, process optimization offers additional energy savings. Multi-stage membrane processes, for example, offer substantial energy advantages over single-stage processes. The first stage operates at a lower pressure and lower recovery rate. The second stage applies a higher pressure to the first stage’s concentrate and achieves a higher recovery rate. Multi-stage systems require more capital investment but are more efficient to operate. Other process-related savings come from advanced sensors that can monitor water levels, pressures, flow rates, and dissolved solids throughout the plant and can signal controls (e.g., valves and motors) to respond in real time. “Larger plants will also benefit from economies of scale,” says McGovern.

Using renewable energy, such as wind and solar, is another appealing strategy. Though costly to develop or install, government agencies offer rebates and incentives for renewables and their operation is inexpensive. Renewables do not reduce energy consumption but may be more environmentally acceptable than conventional fossil fuels. Improvements in wind forecasting and solar batteries are making these resources more reliable. Wind- and solar-rich sites can self-generate some or all of the energy required for desalination.

After water is desalinated, there remains the brine to dispose of. The environmental and ecological impacts associated with brine discharges are not fully understood and must be weighed carefully.

“The drawbacks of desalination are cost and localized environmental impact where brine is discharged to the sea,” says McGovern. Carlsbad’s brine will be diluted before discharge to the ocean, and scientists will monitor conditions for any impacts on marine life.

For inland desalination, there is no ocean disposal option, though others exist. In low volumes and with proper permitting, brine may be discharged to surface waters and municipal sewer systems. Land application can be effective seasonally and over large areas.

Under certain geologic conditions, brine may be injected underground through deep wells, as is done in Texas. State and federal permits are required, including approval from EPA’s Underground Injection Control program. Underground injection is expensive due to the rigorous site evaluations, construction, and maintenance involved.

Another option is evaporation ponds, which are most effective in dry climates. In these surface impoundments brine continues to concentrate as water evaporates into the atmosphere. No federal permit is required, though some states require monitoring. The dried salt can then be used commercially.

Desalination projects are at risk for corrosion and associated problems, such as scaling and biofouling, that accompany saltwater chemistry. The main threats—pitting and crevice corrosion—can be prevented through the use of corrosion-resistant materials. Stainless steel and dozens of other alloys effectively prevent corrosion in common desalination equipment. Selection of corrosion-resistant materials is critical and must be considered carefully during design.

Worldwide, says McGovern, “one of the biggest challenges for desalination is that there is rarely a preexisting regulatory structure in countries that are new to the technology. Developing such regulations requires time and care.” Even where desalination is established, permitting can be an effort in itself. The Carlsbad project required five years of coordination on at least 27 permits, plus dozens of public hearings and technical studies.

Though laborious, Carlsbad’s process has become a regulatory roadmap for future permitting. In May, the State Water Resources Control Board amended its Ocean Plan to streamline permitting for seawater desalination facilities. “Adoption of the Ocean Plan amendment reinforces state water resource management plans and policies that identify the Pacific Ocean as a drinking water resource,” says Scott Maloni, vice president of Poseidon Water, in a news release. “California’s coastal communities can now look with greater certainty to the west for a reliable, environmentally friendly, drought-proof source of drinking water.” WE_bug_web

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